A tuned mass damper, also known as a harmonic absorber, is a device mounted in structures to reduce the amplitude of structural vibrations. The application of such tuned mass dampers can prevent discomfort, damage, or outright structural failure depending on the application. Tuned mass dampers are used to stabilize structures and protect against violent motion caused by harmonic vibration. A tuned mass damper aims at reducing the vibration of a structure with a comparatively lightweight component so that the worst-case vibrations become less intense. Structures employing tuned mass dampers are tuned to either move the main mode away from a troubling excitation frequency, or to add damping to a resonance that is difficult or expensive to damp directly.
Tuned mass dampers are typically attached to a structure at an effective position to counteract the system's vibration. A basic tuned mass damper comprises a mass that is attached to the vibrating structure via a suspension element, which typically consists of a spring and a damper, thus changing the vibrating characteristics of the vibrating structure.
The mass damper is tuned to the vibrating structure such that the mass and the stiffness of the suspension element are selected to provide an appropriate counterforce to the disturbing excitation force. In particular, the mass ratio, i.e. the relativity of mass between the tuned mass damper mass and the vibrating structure, and the tuning frequency of the mass damper may be specifically calculated.
Dampers are frequently used in e.g. wind turbines, power transmission structures, automobiles and buildings which are subjected to vibratory excitations that may cause the structure to vibrate at a predetermined frequency. Vibratory oscillations of these and other structures can cause inaccuracies in equipment associated therewith and fatigue damage to the structures.
Particularly in wind turbines, parts of the wind turbine e.g. a wind turbine tower structure may undergo undesired vibrations, i.e. oscillatory or repeating displacements in any direction (fore-aft vibrations, side-to-side or lateral vibrations, longitudinal vibrations, torsional vibrations, etc.) of different amplitudes and frequencies (high or low, constant or varying) during operation. These vibrations may be caused by different factors, e.g. wind acting on the tower, blades passing along the tower and locally disturbing the wind flow, vibrations transmitted from the gearbox to the tower, rotor movements, nacelle imbalances, vibrations from the hub transmitted to the tower etc.
Additionally, structures of offshore wind turbines are subject to several loads, such as for example impacts, forces exerted by waves, currents and tides. In the presence of such loads offshore wind turbines may have a tendency to destabilize. Particularly, these loads may induce side-to-side motions which may not be properly damped by traditional aerodynamic damping mechanisms such as e.g. pitching. Furthermore, offshore wind turbine towers may be higher, and therefore more prone and sensitive to oscillations, than onshore wind turbine towers.
If a tower is subjected to vibrations during a prolonged period of time, fatigue damage may result. Fatigue damage may lead to a reduced life time of the wind turbine tower and/or its components. Furthermore, a danger exists that when vibrations cause resonance in the wind turbine tower, this can lead to a potentially dangerous increase of the vibrations. A further complicating factor is that the size of wind turbines (rotor, nacelle, tower, etc.) keeps increasing. Also, as towers become higher, the effect of vibrations becomes more critical.
Tuned mass dampers may be placed in confined spaces e.g. the above-commented wind turbine towers. However, these tuned mass dampers can present problems such as large excursions in case of extreme load cases which may lead to an impact of the tuned mass damper with the surroundings. In fact, a possible stroke of the tuned mass damper against the confined spaced where the tuned mass damper is placed is one of the major concerns for the designers. An impact of the tuned mass damper during normal operation or extreme events may damage the tuned mass damper, the surroundings of the tuned mass damper and it may go in detriment of the tuned mass damper performance e.g. detuning the tuned mass damper.